Fuel ether production methods conventionally consist in adding an alcohol to a branched olefin. Examples thereof are the methyl tertiobutyl ether (MTBE) production processes wherein methanol is added to isobutene, ethyl tertiobutyl ether (ETBE) production processes by addition of ethanol to isobutene, as well as processes for producing various ethers such as isopropyl tertiobutyl ether (IPTBE) from isopropanol and isobutene, tertio amyl methyl ether (TAME) from methanol and isoamylene, or ethyl tertioamyl ether (ETAE) from ethanol and isoamylene.
In general terms, all these processes comprise a first reaction section wherein the ether is produced in the liquid phase, at low temperature, by reaction of an iso-olefin with a monoalcohol, in the presence of an acid catalyst, generally of sulfonic polystyrene type in acid form. The reaction is carried out in one or more reactors in series. The reaction is very selective towards the iso-olefins but it is always carried out with excess alcohol so as to cause the chemical ether formation equilibrium to shift. The feed treated is generally a hydrocarbon cut derived from FCC, steam cracking or from a dehydrogenation operation, and it generally contains less than 50 wt. % iso-olefins, the rest consisting of a mixture of hydrocarbons practically inert towards the etherification reaction.
The main reaction section is then followed by a separation stage whose goal is to separate the ether fraction formed, the unreactive or unreacted hydrocarbons for later use, and the excess alcohol. This alcohol is generally recycled to the main reaction section.
The separation section generally consists of a fractionating column that possibly comprises an additional catalytic section intended to push the conversion of the iso-olefins to form ether in larger amounts. It generally allows to collect the ether at the bottom and the hydrocarbon mixture at the top of the column. Division of the alcohol among these two fractions occurs according to the nature of the alcohol and to the composition of the hydrocarbon cut used, and therefore finally according to the nature of the ether produced in the reaction section.
Operation of the fractionating column is generally complex because, in principle, one wants to benefit from the existence of azeotropes between alcohol and ether, alcohol and hydrocarbons, in order to optimize the separation and/or the separation/reaction in a reactive distillation column when the goal is to maximize the production of ether, as described for example in patent applications FR-2,675,055 A1 and FR-2,678,846 A1.
Patent FR-2,683,523 aims to wash the alcohol-ether cut obtained at the bottom of the fractionating column with water. In addition to the difficulty linked with the recovery of the alcohol through a sequence of columns, the technique is penalized by the production, on the one hand, of a water-saturated ether requiring later treatment and, on the other hand, of a water-laden alcohol.
When the alcohol is not methanol, since for the latter there is no azeotrope formation, various methods of recovering the alcohol contained in the ether in the bottom of the separation section have already been proposed.
Patent FR-2,672,048 provides an alternative to patent FR-2,683,523 by taking advantage of the variation, with the pressure, of the composition of the azeotrope of the alcohol-ether mixture. Using two distillation columns operating at two different pressures allows to obtain the ether in the bottom of the first column operated at high pressure and the alcohol in the bottom of the second column operated at low pressure. Using an azeotrope-generating agent in order to facilitate separation of the ether and of the alcohol according to the latter technique is described in patent FR-2,673,624. This technique has the drawback of being investment costly and of recycling with the alcohol various impurities present in the ether cut from the synthesis stage. Recycling these impurities leads to their progressive accumulation that may eventually disturb the proper operation of the process.
Patent FR-2,719,581 aims to achieve separation of the various compounds from the reaction section by distillation, with a first column supplied with the alcohol-ether-hydrocarbon mixture allowing to recover the hydrocarbons at the top of the column and the purified ether at the bottom of the column, and a second column supplied by lateral withdrawal from the first column, for which the alcohol is collected in the bottom and an alcohol-ether-hydrocarbon mixture is collected at the top and recycled to the first column.
The different techniques presented above have in common the fact that they produce at the top of the first distillation column an alcohol-rich hydrocarbon cut. The solution that is generally selected for collecting this alcohol consists in washing this hydrocarbon cut with water. The first drawback thereof is that a water-saturated hydrocarbon cut is obtained, the second one is that it requires using a distillation column for the water-alcohol mixture thus obtained. This distillation is furthermore generally penalized by the formation of an azeotrope between the alcohol and the water.
In the case of methanol that forms no azeotrope with water, various alcohol recovery methods are provided, but the drawback then lies in the methanol concentration of the liquid water-methanol effluent obtained. Separation of the water and of the methanol can be carried out by distillation, but this technique is generally energy costly, or by stripping with a water-saturated gas as described in patents EP-362,023 and EP-783,031. The latter technique however has limits in terms of recoverable methanol amount.
Consequently, except for methanol, the various techniques presented above lead to recycle to the reaction section a highly water-laden alcohol. In general, the water content can be up to 10% by weight in the case of ethanol, up to 30% by weight in the case of a C3 alcohol and 45% by weight in the case of a C5 alcohol. When the highly water-laden alcohol is recycled to the reaction section, deactivation of the resins is observed through decrease of their acidity, as well as the formation of unwanted alcohols resulting from the addition of water to the branched olefins instead of the desired reaction consisting in the addition of these alcohols to the branched olefins.
An alternative to the extraction of alcohol by water consists in using a non-aqueous ionic liquid. Arce et al. (Ind. Eng. Chem. Res. 2004, 43, 8323) have assessed this solution for extracting the ethanol contained in tert-amyl ethyl ether (TAEE) by 1-butyl-3-methylimidazolium trifluoromethanesulfonate. The results obtained imply the possibility of extracting the alcohol contained in the ether, but at the cost of a high co-absorption of ether in the extraction solvent. The lack of selectivity of the ionic liquid selected does not favour an economical use of this extraction solvent.
Besides, Arce et al. (Chemical Engineering Journal 115 (2006) 219-223) have also investigated the possibility of using this solvent for extracting the ethanol contained in ETBE. According to Arce et al., the ionic liquid is therefore used to separate the alcohol from the ether produced.